remove checkpoint_version read gates

Drop the EnsureCanReadVersion gates at resume/explain/rewind/export and the unreachable IsUnsupportedVersion error branches behind them. The gates were forward-compat tripwires for checkpoint formats that never shipped; checkpoint_version is a pure label nothing dispatches on.

Also drop two ReadCheckpoint calls that existed only to feed the gate — ReadRawSessionLogForCheckpoint already reads the checkpoint itself.